📚 How Many Protons, Neutrons and Electrons? | 如何计算质子、中子和电子数？
Counting subatomic particles is one of the most basic but frequently examined skills in Cambridge A Level Chemistry. You must be able to use atomic number (Z), mass number (A) and ionic charge to state the number of protons, neutrons and electrons in atoms, ions and isotopes.
计算亚原子粒子数是剑桥 A Level 化学中最基础但高频考查的技能之一。你必须能够利用原子序数（Z）、质量数（A）和离子电荷，说出原子、离子和同位素中的质子数、中子数和电子数。
1. Why This Skill Matters | 为什么这项技能很重要
Subatomic particle counting appears in almost every early topic of A Level Chemistry: atomic structure, isotopes, mass spectrometry, ionic bonding, mole calculations and even redox. If you cannot state the number of protons, neutrons and electrons quickly and accurately, later calculations become unreliable.
亚原子粒子计数几乎出现在 A Level 化学早期所有主题中：原子结构、同位素、质谱、离子键、物质的量计算甚至氧化还原。如果你不能快速准确地写出质子数、中子数和电子数，后续计算就容易出错。
Examiners often test this skill indirectly. A question may ask you to explain why an ion has a particular charge, why two isotopes have the same chemical properties, or why the mass spectrum of an element shows several peaks. All of these depend on correct particle counting.
考官常常间接考查这项技能。题目可能要求你解释为什么某种离子带有特定电荷，为什么两种同位素化学性质相同，或者为什么某元素的质谱出现多个峰。这些都依赖正确的粒子计数。
The good news is that the method is highly systematic. Once you know the nuclear symbol and the charge, you can always work out the three numbers in a fixed order.
好消息是这种方法非常有系统。只要知道核素符号和电荷，你总可以按固定顺序算出这三个数。
2. Nuclear Symbols and the Meaning of Z and A | 核素符号以及 Z 与 A 的含义
A nuclide is written with the mass number A as a superscript on the left of the element symbol, and the atomic number Z as a subscript on the left. This is often called the AZX notation or nuclide notation.
核素写作元素符号左侧上标质量数 A、下标原子序数 Z。这种写法通常称为 AZX 符号或核素符号。
- Z = atomic number = number of protons in the nucleus.
- Z = 原子序数 = 原子核中的质子数。
- A = mass number = total number of protons + neutrons.
- A = 质量数 = 质子数 + 中子数。
e lower number is always the atomic number, and the upper number is always the mass number.
例如，¹²₆C 的 Z = 6，A = 12。下方数字始终是原子序数，上方数字始终是质量数。
In printed exam papers, the atomic number may sometimes be omitted because the element symbol already identifies Z. If the symbol is given, you can look up Z from the periodic table. If both numbers are given, always check which is which before calculating.
在考试印刷材料中，原子序数有时会省略，因为元素符号本身已经确定了 Z。如果给出元素符号，你可以从周期表中查 Z。如果两个数字都给出，计算前一定要确认哪个是哪个。
3. Protons: Always from the Atomic Number | 质子：始终由原子序数确定
The number of protons is equal to the atomic number Z. This value is fixed for a given element and does not change when the atom forms an ion or when the isotope changes.
质子数等于原子序数 Z。对于给定元素，这个数值是固定的，不会因为形成离子或同位素改变而改变。
ery chlorine atom, chloride ion or chlorine isotope contains 17 protons. Changing the number of protons would change the identity of the element.
例如，氯的 Z = 17，所以每一个氯原子、氯离子或氯同位素都含有 17 个质子。改变质子数会改变元素的种类。
protons = Z
This is the simplest step, so always write down Z first before moving to neutrons and electrons. In a multi-step question, it also gives you a secure starting point.
这是最简单的一步，因此总是先写下 Z，再计算中子数和电子数。在多步题目中，它也为你提供一个可靠的起点。
A common mistake is to read the mass number as the proton number. Remember that the larger number is not always the proton number; the atomic number is defined by the element itself.
常见错误是把质量数当作质子数。记住，较大的数字不一定是质子数；原子序数由元素本身决定。
4. Neutrons: Mass Number Minus Atomic Number | 中子：质量数减去原子序数
The mass number A counts all nucleons, which are the protons and neutrons in the nucleus. Since protons are counted by Z, the neutron number N is obtained by subtracting Z from A.
质量数 A 计算所有核子，即原子核中的质子和中子。由于质子数由 Z 给出，中子数 N 通过用 A 减去 Z 得到。
neutrons = A − Z
= 14 − 6 = 8. The two isotopes have the same proton number but different neutron numbers.
对于 ¹²₆C，中子数 = 12 − 6 = 6。对于 ¹⁴₆C，中子数 = 14 − 6 = 8。两种同位素质子数相同，但中子数不同。
n count, not the nucleus. Therefore ²³₁₁Na and ²³₁₁Na⁺ both contain 23 − 11 = 12 neutrons.
在离子中，中子数不变。原子通过得到或失去电子而带电，只影响电子数，不影响原子核。因此 ²³₁₁Na 和 ²³₁₁Na⁺ 都含有 23 − 11 = 12 个中子。
the equation to find Z: Z = A − N. This is useful when the element symbol is not provided.
如果题目给出中子数和质量数，你可以重排方程求 Z：Z = A − N。当没有给出元素符号时，这很有用。
5. Electrons: Neutral Atoms versus Ions | 电子：中性原子与离子的区别
r of electrons equals the number of protons. Therefore electrons = Z for any neutral atom.
在中性原子中，电子数等于质子数。因此对于任何中性原子，电子数 = Z。
When the particle is an ion, the charge tells you how many electrons have been gained or lost. A positive charge means electrons have been lost; a negative charge means electrons have been gained.
当粒子是离子时，电荷告诉你得到了或失去了多少电子。正电荷表示失去了电子；负电荷表示得到了电子。
electrons = Z − charge
harge +1, so electrons = 11 − (+1) = 10. Cl⁻ has charge −1, so electrons = 17 − (−1) = 18.
这里的电荷必须包含符号。例如，Na⁺ 的电荷为 +1，所以电子数 = 11 − (+1) = 10。Cl⁻ 的电荷为 −1，所以电子数 = 17 − (−1) = 18。
- Neutral atom: electrons = protons.
- 中性原子：电子数 = 质子数。
- Cation: electrons = protons − charge.
- 阳离子：电子数 = 质子数 − 电荷数。
- Anion: electrons = protons + size of charge.
- 阴离子：电子数 = 质子数 + 电荷数大小。
Always count electrons last, after you have confirmed protons and neutrons. This order reduces mistakes in multi-step calculations.
始终在确认质子数和中子数之后，最后计算电子数。这个顺序可以减少多步计算中的错误。
6. Charge Rules for Ions | 离子电荷规则
Main-group ions often form charges related to their group number. Metals on the left of the periodic table tend to lose electrons to form positive ions, while non-metals on the right tend to gain electrons to form negative ions.
主族离子通常形成与其族数相关的电荷。周期表左侧的金属倾向于失去电子形成正离子，而右侧的非金属倾向于得到电子形成负离子。
| Ion | Charge | Protons | Electrons |
| Na⁺ | +1 | 11 | 10 |
| Mg²⁺ | +2 | 12 | 10 |
| Al³⁺ | +3 | 13 | 10 |
| Cl⁻ | −1 | 17 | 18 |
| O²⁻ | −2 | 8 | 10 |
| N³⁻ | −3 | 7 | 10 |
The table shows an important pattern: Na⁺, Mg²⁺, Al³⁺, O²⁻, N³⁻ and F⁻ all have 10 electrons. They are isoelectronic with neon. Recognising such patterns helps you check your answers quickly.
该表显示一个重要规律：Na⁺、Mg²⁺、Al³⁺、O²⁻、N³⁻ 和 F⁻ 都有 10 个电子，与氖等电子。识别这类规律有助于快速检查答案。
Transition metal ions can have variable charges, so you must use the charge given in the formula or name. For example, Fe²⁺ has 24 electrons, while Fe³⁺ has 23 electrons.
过渡金属离子可以有可变电荷，因此必须使用化学式或名称中给出的电荷。例如，Fe²⁺ 有 24 个电子，而 Fe³⁺ 有 23 个电子。
7. Isotopes: Same Protons, Different Neutrons | 同位素：同质子不同中子
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same atomic number Z but different mass numbers A.
同位素是同一元素的原子，质子数相同但中子数不同。因此它们原子序数 Z 相同，但质量数 A 不同。
17 protons. The first has 35 − 17 = 18 neutrons, and the second has 37 − 17 = 20 neutrons.
对于氯，两种常见同位素是 ³⁵₁₇Cl 和 ³⁷₁₇Cl。两者都有 17 个质子。前者有 35 − 17 = 18 个中子，后者有 37 − 17 = 20 个中子。
- Same Z: same chemical properties, because chemistry depends mainly on electrons.
- Z 相同：化学性质相同，因为化学性质主要取决于电子。
- Different A: different physical properties such as density and rate of diffusion.
- A 不同：密度、扩散速率等物理性质不同。
- eutral isotopes have the same electron count, because electrons = Z for each neutral atom.
- 中性同位素电子数相同，因为每个中性原子的电子数 = Z。
If an isotope forms an ion, the charge still determines the electron count. For example, ³⁵₁₇Cl⁻ and ³⁷₁₇Cl⁻ both have 18 electrons.
如果同位素形成离子，电荷仍然决定电子数。例如，³⁵₁₇Cl⁻ 和 ³⁷₁₇Cl⁻ 都有 18 个电子。
8. Worked Example: Step-by-Step | 分步例题
Question: State the number of protons, neutrons and electrons in ²⁷₁₃Al³⁺.
题目：写出 ²⁷₁₃Al³⁺ 中的质子数、中子数和电子数。
Step 1: Identify Z from the subscript. Z = 13, so protons = 13.
步骤 1：从下标确定 Z。Z = 13，所以质子数 = 13。
Step 2: Identify A from the superscript. A = 27, so neutrons = A − Z = 27 − 13 = 14.
步骤 2：从上标确定 A。A = 27，所以中子数 = A − Z = 27 − 13 = 14。
he charge to find electrons. The charge is +3, so electrons = Z − charge = 13 − (+3) = 10.
步骤 3：利用电荷求电子数。电荷为 +3，所以电子数 = Z − 电荷 = 13 − (+3) = 10。
²⁷₁₃Al³⁺: 13 protons, 14 neutrons, 10 electrons
6, A = 32, charge = −2. Protons = 16, neutrons = 32 − 16 = 16, electrons = 16 − (−2) = 18.
第二个例子：³²₁₆S²⁻ 的 Z = 16，A = 32，电荷 = −2。质子数 = 16，中子数 = 32 − 16 = 16，电子数 = 16 − (−2) = 18。
Notice that the same three-step method works for atoms, cations and anions. The only difference is the charge value used in the electron calculation.
注意，同样的三步法适用于原子、阳离子和阴离子。唯一区别是电子计算中使用的电荷值。
9. Quick Reference Table and Checklist | 速查表与自检清单
Use this table to organise your working. It summarises the key relationships and helps you avoid mixing up protons, neutrons and electrons.
用这个表组织你的解题过程。它总结关键关系，帮助你避免混淆质子、中子和电子。
| Quantity | Formula | Notes |
| Protons | Z | Fixed for the element |
| Neutrons | A − Z | Changes between isotopes |
| Electrons | Z − charge | Use the sign of the charge |
| Mass number | protons + neutrons | Always a whole number |
| Charge | protons − electrons | Positive if electrons lost |
Checklist before you write your final answer: Have you used the correct Z? Have you subtracted Z from A for neutrons? Have you included the sign of the charge for electrons? Have you kept the neutron number unchanged for ions?
写出最终答案前的自检清单：你用了正确的 Z 吗？你用 A 减 Z 求中子了吗？你在电子计算中包含了电荷符号吗？你对离子保持中子数不变了吗？
If all four answers are yes, your particle count is almost certainly correct.
如果四个答案都是肯定的，你的粒子计数几乎肯定正确。
10. Common Pitfalls and Exam Traps | 常见陷阱与考试失分点
One common error is to use the mass number as the proton number. This happens when the student reads the superscript first and forgets that Z is the atomic number.
一个常见错误是把质量数当作质子数。这发生在学生先读上标，却忘记 Z 是原子序数的时候。
Another error is to add electrons for a positive charge. A positive charge means the particle has lost electrons, so the electron number is smaller than Z. Negative charge means electrons are gained, so the electron number is larger than Z.
另一个错误是正电荷时加电子。正电荷意味着粒子失去了电子，所以电子数小于 Z。负电荷意味着得到电子，所以电子数大于 Z。
- Wrong: Na⁺ has 12 electrons. Correct: Na⁺ has 10 electrons.
- 错误：Na⁺ 有 12 个电子。正确：Na⁺ 有 10 个电子。
- Wrong: ¹⁴₆C has 14 neutrons. Correct: ¹⁴₆C has 8 neutrons.
- 错误：¹⁴₆C 有 14 个中子。正确：¹⁴₆C 有 8 个中子。
- Wrong: Cl⁻ has 16 electrons. Correct: Cl⁻ has 18 electrons.
- 错误：Cl⁻ 有 16 个电子。正确：Cl⁻ 有 18 个电子。
Also be careful with notation. The superscript is the mass number, the subscript
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